The polypites which digest the food are vermiform double sacs communicating at one end by a valve with the canal in the body of the animal; and at the free end they are prolonged into a mouth with an everted lip, and the digesting apparatus lies in the centre. Each polypite is supplied with food by its own fishing-line descending from a point close to where the polypite is fixed to the long axis. It is a long, tubular, branched tentacle, each branch ending in a coloured, pear-shaped, or fusiform battery of thread-cells with their stings. A gelatinous plate is placed on the upper side of the common axis immediately over the isolated groups, to protect and separate them.
Such are some of the most general characters of the family Diphyidæ: the Praya diphys has something peculiar to itself.
In the Praya, each individual group has a swimming-bell of its own adjacent to the polypite, and lying parallel to the axis of the animal, with its mouth turned backwards. It is connected by tubes both with the general central canal, and with a helmet-shaped protecting plate. On the other side of the polypite, there is a tuft of vermiform buds with spiral terminations, bristled with thread-cells. From the centre of this tuft a tentacle, or fishing-line, descends with numerous branches, the whole forming a tubular system connected with the common canal in the axis. Each of the branches of the tentacle terminates in a vermilion-coloured tendril, coiled up into a minute capsule. The inside of the tendril is not only bristled with the points of sabre-shaped darts, but it conceals a filament crowded with thread-cells. On the slightest touch, the tendril stretches out like a corkscrew of red coral, and every dart springs forth. Such is, more or less, the complicated structure of the offensive and defensive weapons of many of this order of oceanic Hydrozoa, which appear to the naked eye as merely brightly-coloured points. The use of these tentacles, or fishing-lines, is the same in all; they seize, kill, and carry their victims to the mouth of the polypite by contracting their long lines.
In the Praya, the groups are individualized in the highest degree consistent with union; for, when the animal is at rest, each of the individual groups, amounting to thirty or forty, swims about by means of its little bell independent of the rest. Their motions can be compared to nothing but a troop of jugglers performing gymnastic exercises round a cord represented by the common body of the animal; except for adherence to which the life and will of each group are so perfectly independent, that the mutual dependence of the whole is only seen when the common trunk contracts to bring all its appendages towards the two principal bells, which then begin to move.[[25]]
Thus each group has a special life and motion, controlled by a general life and motion; strong individual muscular power controlled by general muscular power; yet no nervous system has as yet been discovered, so this animal activity must for the present be attributed to a strong, inherent, contractile power in the muscular fibre. The Praya is seldom complete, on account of the ease with which it casts off its great bells.
None of the Diphyidæ have special organs for respiration; their juices are aërated through their delicate tissues. They are diœcious, and invariably produce perfect male and female medusiform zooids; they are situated among the groups of the polypites and their appendages, and are attached to the axis of the animal. When free, they swim away by the contraction of their bells; the eggs are fertilized, and produce young Diphyidæ, male and female; so these animals, like most of the oceanic Hydrozoa, have two alternate stages of existence.
Physograde Acalephæ.
The Galeolaria lutea ([fig. 118], frontispiece) is similar to the Praya diphys in having a slender, tubular body, with groups of sterile polypites and their appendages hanging at intervals along its under-side like a fringe, and also in having two swimming-bells at its anterior extremity; but it has no special small bells. The large ones differ from each other in size, form, and position. The largest is nearly cylindrical, its mouth is turned upwards, and its rim is elevated at one part into two stiff organs like the blinkers that are put over horses’ eyes: besides these, it has six salient points, which nearly close the mouth of the bell at each contraction of the muscular iris that lines the margin of the cavity. The small bell, which goes first in swimming, is thicker and shorter, and its side rises in a hump, upon which the closed end of the large bell rests, and in the cavity between the two the anterior extremity of the filiform body is fixed. Each of the groups of polypites, with their tentacles, lies immediately under its spathe-shaped protecting plate. The polypites are very contractile, and on their protuberant part, containing the digestive cavity, there is a large circular space, which, as well as the whole tissue of the polypite and the stinging capsules at the extremities of the tentacles, are of an orange colour, and are akin in structure to those described.
When very young the Galeolaria and its congeners have only their swimming-bells and one polypite group affixed to the end of a short tubular axis; by and by a second group is developed from buds between the bells and the first group; then a third is developed between the bells and the second group, and so on; the length of the body and the number of groups continue to increase indefinitely. It is only when the animal is full grown and complete in all its parts, that reproductive organs are developed towards its posterior end. Buds then appear upon the hollow stems of the polypites towards the posterior end of the body. But as the Galeolaria is diœcious, male and female buds are never on the same individual. The female buds become medusiform zooids, like those of the Praya diphys, only the transparent cup, with which it swims away from its parent, has two projections like ears on its rim.
The development of the buds in the male Galeolaria is similar. At first they are pale, but they assume an orange red colour as they advance towards maturity, and, when complete, the sac which hangs down from the centre of the transparent cup becomes of a brilliant vermilion. These male and female medusa-zooids swim about for several days, and the fertilized eggs are hatched into young Galeolariæ, male and female.